The invention relates to a thermal insulating flexible ceramic based on a microporous oxide aerogel obtained from flame hydrolysis, with elastically bendable or limply bendable inorganic fibers and possibly additional additive substances, such as especially opacifiers, as well as a process for its production, a foil made from it and a laminated thermal insulating element produced therewith.
Thermal insulating flexible ceramics based a microporous oxide aerogel obtained from flame hydrolysis, especially silica aerogel, have been known widely and are on the market for example under the tradename MINILEIT (registered trademark) of the assignee and have been described often.
As the German Pat. No. 19 54 992 explains in detail in this connection, the essential problem consists in the fact that plastics made from such finely dispersed substances have to be sure excellent thermal insulation characteristics but are hardly capable of a mechanical stress. Whenever such a substance is pressed into a plate, as is known for example from the German OS No. 30 04 187, for the formation of the support plate for an electric heating coil of a radiation heat cooking plate, then even in the case of slight mechanical stresses, the microporous material crumbles on the surface and the plate will be so brittle that it will break almost without previous deformation in the case of the slightest stress from bending. Therefore, the mechanical characteristics of such plates resemble those of, for example, bound fine sand, etc. Such plates would be suitable therefore only for cases of application in the case of which the material is practically not exposed to any kind of mechanical stresses, thus, lies say only in the receiving shell for the heating coil of a cooking plate and mounts the former.
According to the doctrine of the cited German Pat. No. 19 54 992, such a flexible ceramic is to obtain a better mechanical load carrying ability through the fact that it be surrounded by a flexible casing which fits against the flexible ceramic under pressure. According to the further doctrine of e.g. the German Pat. No. 20 36 124, the pressing of the microporous substances in the casing is to be carried out in such a way that an as close as possible meshing of the finely dispersed substances among themselves and with the surface or the pores of the relatively rough encasing, for example, made of glass fiber, will take place. Just as in the case of the German Pat. No. 19 54 992, an increased rigidity is to be imparted with this process to said sandwich plate, but an airtight sealing and evacuation as well as a use of resin-like binders which decrease the capacity for thermal insulation, is to be avoided. The rigidity thus increased of the encased plate to be sure raises the capacity for withstanding bending stresses of the plate to a certain even though relatively slight degree, but the material within the encasing will break practically without any previous deformation as soon as a stress occurs exceeding the flexural strength.
From the German OS No. 29 28 695 therefore, a thermal insulation plate basically corresponding in its structure to the plate of the German Pat. No. 20 36 124 has been known, in the case of which there is avoided by a separating agent between the surface of the finely porous damping material and the encasing, that the pressing process in this case leads to a mechanical clawing together. As a result of that, relative movements between the surface of the microporous flexible ceramic and the encasing become possible and it results, which per se is surprising, in a considerably improved bendability of the compound body, but only as long as the microporous flexible ceramic originating from the pressing process, is held in a state of pressure on all sides in the encasing through the tensile stresses.
Thus, with such encasings, it is true that the mechanical properties of thermal insulating plates etc. on the basis of such microporous flexible ceramics may be improved to a certain extent, however, quite apart from the necessary expenditures for the encasing this improvement is only sufficient for a relatively small number of cases of application for such a highly effective thermal insulating flexible ceramic and very many other cases of application are eliminated by the necessary encasing made of a fiber glass fabric, etc., i.e., in the cases in which such an encasing disturbs or cannot be used at all.
In regard to the material composition of such a flexible ceramic in detail as well as with regard to its physical properties, the status of the prior art offers a very large number of proposals. According to the German AS No. 16 71 186, for example, the flexible ceramic is to consist of 70% by weight of a silica-aerogel, 20% by weight of channel black as an opacifier and 10% by weight of aluminum silicate fibers, and it is to be condensed to a density or a weight per unit of volume of 240 kg/m.sup.3, so that at a temperature of 200.degree. C., it will result in a heat conductive capacity of the flexible ceramic of 0.022 W/mK. According to the doctrine of the German AS No. 16 71 186, the bulk density may be increased if necessary also up to 400 kg/m.sup.3, but we should like to point out that too high a pressing with a compression of the microstructure of the material is to be avoided, since any further compression of such a structure would increase the heat conductive capacity of the material, and insofar as it attempts to destroy the microporous structure also would increase the conductive factor of the fiber material by increasing the overall contact surface between the participating particles and the insulation. The aluminum silicate fibers which may be wholly or partly replaced by carbon fibers increase to be sure the conductivity of the substance but they form a net of fibers which binds the aerogel and opacifier particles loosely.
According to the German OS No. 28 06 367, the material is to contain aluminium oxide fibers in a quantity of up to 12% by weight. In the case of the upper limit of the portion of fibers of 20% stated there in one place, we are dealing with an obvious typing error, particularly since it has also been expressly explained that approximately 12 and preferably 10% by weight of aluminum oxide fibers constitute a practical upper limit, while a preferred portion of aluminum oxide fibers that are to be added lies in the area between 1 and 7% by weight of the entire material. The upper limit of the portion of aluminum oxide fibers of 10 to 12% does not only result in consequence of the heat conductive capacity rising with a higher fiber portion, but also is a result of problems of mechanical strength of the material, which however, is to be used again anyway for the mounting of the electric heating coil of a hot plate and for similar cases of application in case of which mechanical stresses hardly occur. The bulk density of the flexible ceramic according to the doctrine of the German OS No. 28 06 367 may lie between 160 and 480 kg/m.sup.3, but in the case of the therein stated data of 0.16 to 0.48 kg/m.sup.3, we are dealing again with an obvious clerical error as far as the decimal point is concerned.
From the European patent application OS No. 13 387 such a flexible ceramic has been known furthermore which may contain 30 to 95% by weight of aerogel and 5 to 70% by weight of opacifier, possibly with the addition of up to 5% by weight of organic or inorganic binder, whereby the opacifier is admixed to the agglomerating aerogel for coagglomeration in immediate succession to the flame hydrolysis in order to avoid the expenditure of a mechanical mixing of these substances and in order to achieve improved mechanical characteristics. At the same time and if necessary, the opacifier may show anisotropic geometrical shapes, such as for example fiber or flake structure. This mixture is compressed with a pressure of for example 15 bar which could lead to a bulk density in the order of magnitude of about 250 kg/m.sup.3. Beside the coefficient of thermal conductivity, the indentation hardness was measured in samples produced thus, whereas the same values were determined in comparative examples which were materially the same and in the case of which a mechanical mixing of the aerogel with the opacifier was accomplished. While no determinations were made concerning bending strength of the samples, the examples and comparative examples show that the indentation hardness in the case of the coagglomeration would lie higher only whenever ilmenite was used as an opacifier so that in this respect too, an increase of the mechanical strength may possibly be achieved in exceptional cases or in a very limited way. By coagglomeration alone instead of mechanical mixing, an increase of the bending strength or bendability of the flexible ceramic is not to be expected either.
From the U.S. Pat. No. 3,055,831 finally, a great multiplicity of examples for material mixtures and mechanical treatments of such flexible ceramic has been known, whereby the bulk densities vary between about 95 and 800 kg/m.sup.3 and between 2 and 75% of reinforcing fibers added. At the same time, however, an organic (or also inorganic) binder such as phenolformaldehyde-resin was added in a quantity which results in a desired strength; as a result of that, it is true, one may achieve any desired strength in case of correspondingly high binder content and good binding action, especially of organic binders, but naturally at the expense of the thermal insulating characteristics and frequently with energy consuming curing. In the case of very high fiber contents of 38 and 67%, a moldable mat developed with low bulk density below 200 kg/m.sup.3 which to be sure is easily bendable, but has in practice no bending strength and tensile strength, as has been known per se in the case of fiber mats. In the case of a fiber content of 15% and a synthetic resin binder in a portion of 10%, in each case related to the weight, there resulted when using 75% of channel black however without any aerogel, a manageable body with a volumetric density of about 250 kg/m.sup.3 about the strength characteristics of which however there are no detailed data. In the case of a fiber content of 12% and a plastic on the basis of silica aerogel, there resulted in the case of a synthetic resin binder content of 5%, a bulk density of about 320 kg/m.sup.3 with a bending strength of about 52 N/cm.sup.2, in the case of 67% silica aerogel and 16% silicon as additional components or of about 32 N/cm.sup.2 in the case of 55% silica aerogel and 28% titanium dioxide as additional components. A bending strength of 31 N/cm.sup.2 was also achieved with 5% fibers, 81% silica aerogel and 9% black as well as 5% phenol resin binder even in the case of a lower bulk density of about 256 kg/m.sup.3.
The examples stated in the U.S. Pat. No. 3,055,831 all have organic binders in the form of phenol formaldehyde resin or insofar as the above described examples are concerned, they have phenol resins which in the case of the hardening heat treatment, is burnt out practically free of residue in an exothermic process and leads to a sinterlike baking of the finely dispersed particles of the flexible ceramic. The proposal has also been made already to use boron carbide as an inorganic binder which at about 800.degree. C. in an exothermic reaction passes over into boron oxide which bakes the finely dispersed particles together like a cement matrix. The result of such organically or inorganically "hardened" flexible ceramics of this type is in any case a very considerable increase of the bending strength as compared to the same flexible ceramic without binder and generally of the mechanical strength, but the flexible ceramic just as before, is very brittle; it will not yield either noticeably even under higher bending stresses below the bending strength, and after exceeding the bending strength, it will break as a result of the bending stresses suddenly almost without previous deformation.
Beside the very considerable expenditure in production technology as a result of the heat treatment required as a rule over an extended period of time, the customary binder admixtures in the case of flexible ceramics of the present variety have the disadvantage of a relatively inhomogeneous, locally variable hardening. The particles of the binder are ground in special mills, for example, vibrating or ball mills, to a small grain size which should correspond at least approximately to the grain size in the raw material mixture of the flexible ceramic, and are worked mechanically into the raw material mixture with the help of a mixing apparatus. This mixing however is imbued with considerable difficulties, since both the fine grained binder particles as well as the aerogel and opacifier particles immediately form secondary agglomerates during the mixing, so that an effective intermixing of binders and raw materials of the residual mixture will be prevented. Therefore, after hardening, only certain particle compositions will be held together by the binder-agglomerate and the product in the case of stress is inclined to premature breaks in areas which have little binder.
This disadvantage to be sure will be avoided by the doctrine of the German patent application No. 29 42 087.9 of the applicant, according to which the binder, which is distributed evenly with a dispersing agent in a premixture, is present in a finely dispersed manner, so that the aerogel and opacifier particles are cross-linked homogeneously in the compound with their edges and corners. Here, too, however a body develops with to be sure high bending strength, but with extremely low bendability or deflection in the case of bending stress.
The share of fibers and the bulk density play no essential role in the case of such flexible ceramics hardened with binder, since the desired improvement of the mechanical strength, especially of the bending strength, is derived in consequence of the portion of binder from the aerogel and opacifier particles baked together with one another, which form a strong and rigid compound which primarily is held by the sintering together of the particles and not by the occluded fibers. This becomes clear also through the fact that in the case of the above cited examples of the U.S. Pat. No. 3,055,831, no loss in bending strength was found despite a decrease of the portion of fibers from about 12 to 5% and despite a simultaneous decrease of the bulk density from about 320 to 256 kg/m.sup.3. This finding is obviously because the portion of finely dispersed opacifiers and especially of aerogel particles was increased by the decreased portion of fibers resulting in increased sintering and baking together forces, which balance out any possible loss of strength by a decrease of the bulk density.
According to the doctrine of the U.S. Pat. No. 3,055,831, deformable fiber mats should have a fiber portion of at least 35% by weight up to 75% by weight, whereby in the case of a fiber content of 50% by weight, coefficient of thermal conductivity of about 0.036 W/mK is cited. For strong and rigid bodies on the other hand, the fiber content should lie preferably at about 5% by weight but it may rise up to 12 to 15% by weight without however--as explained previously--bringing noticeable strength improvement of the hardened plate in the case of a fiber portion increased to 12 to 15% by weight.
According to the doctrine of the U.S. Pat. No. 3,055,831, asbestos fibers are to be used also at a length which is to lie at least 25% above approximately 6 mm. Such fibers with a mean length between for example 5 and 10 mm may be mixed in any case up to a weight portion of 10 or 12, in the extreme case also 15%, with the aerogel and the opacifier still without causing problems, i.e., without forming fiber agglomerates. The thickness of the fibers according to the U.S. Pat. No. 3,055,831 lies below 20 .mu.m, preferably at 10 .mu.m. Since the diameters of the aerogel particles lie in the order of magnitude of 10.sup.-10 m, even short, thin fibers will still form macroscopic bodies in the microstructure of the aerogel particles. Even in the case of a fiber thickness of a few .mu.m, this is still greater by the factor of 10,000 than the diameter of the aerogel particles.
In summary therefore, the following proposals have been known from the state for the prior art, of improving the mechanical characteristics of flexible ceramics of this kind:
1. The flexible ceramic is pressed in an encasing made of air permeable glass fiber fabric or something similar (German Pat. No. 19 54 992). At the same time, either a clawing together or a clamping together of the flexible ceramic surface with the encasing fabric is achieved (German Pat. No. 20 36 124), in order to improve the reciprocal action and in order to obtain an increased rigidity and bending strength, or a separating agent is inserted between the surface of the flexible ceramic and the encasing (German OS No. 29 28 695), in order to make possible relative movements and in order to make the compound body bendable to a certain extent. Such encasings however are expensive and they restrict the usability of such a thermal insulating material. PA1 2. The mechanical characteristics are to be improved by coagglomeration of the opacifier with aerogel following immediately after the flame hydrolysis (European OS No. 13 387). Even in so far as these mechanical characteristics relate to the hardness of the surface, an improvement may only be achieved in the case of certain opacifiers. An increase in the bending strength or in the bendability of the flexible ceramic by this method has not been stated and is not to be expected either. PA1 3. By the addition of binders, a hardening effect may be achieved (U.S. Pat. No. 3,055,831 or German patent application P No. 29 42 087.9). In the case of a fiber portion of preferably 5 to 10%, a rigid plate of considerable bending strength will result which however is very brittle and may practically not be bent at all. The portion of the fibers, the length of which at 25% or more lies above approximately 6 mm, may be increased to 12 up to a maximum of 15%, however, this is without any measurable influence on the bending strength. A better bendability will likewise not result, since the binder improves the bending strength of the flexible ceramic at any rate. PA1 As a result of a higher increased fiber portion of 35 to 75% by weight, preferably 50% by weight, there results when using about 5% by weight of phenol formaldehyde resin as a binder, a fibrous feltlike body with a low bulk density of below 200 kg/m.sup.3 with a coefficient of thermal conductivity which still lies just barely below that of still air. Such fiber mats are bendable but they have very low strength, especially very low bending strength (Examples IV and V of the U.S. Pat. No. 3,055,831).